Metals to be detected using such an alternating magnetic field are generally classified as either magnetic material or non-magnetic material, and it is known that these two types of materials have different magnetic detection sensitivities (hereinafter referred to as ‘frequency sensitivity behavior’) to frequencies. Furthermore, food packages as a test material includes paper, resin, and even metal such as aluminum, and these packaging materials each have frequency sensitivity behavior. Moreover, food itself contains salt, water, or the like, for example, so they also have respective frequency sensitivity behaviors. However, for the metal detection device, only metals mixed in the food should be detected at an appropriate sensitivity appropriate for respective frequency sensitivity behavior of the magnetic material or non-magnetic material, and also at a sensitivity that enables distinguishing from the frequency sensitivity behavior of the food and packaging material (See Patent Document 1).
Some prior art metal detection devices create an alternating magnetic field alternately in a time-sharing mode to apply the alternating magnetic field of plural appropriate frequencies. For example, the magnetic field by frequency F1 for detecting the magnetic material and the magnetic field by frequency F2 for detecting non-magnetic material are alternately applied via a transmission coil (so-called antenna coil) to the food in a time-sharing mode every time T, and the signal received by a reception coil via the food is branched into frequency F1 and frequency F2 to process frequency F1 element and frequency F2 element, respectively, in order to individually judge whether magnetic material or non-magnetic material is present (see Patent Document 1, for example).
In this case, the magnetic field is driven for independent processing per frequency, so it is easy to select and set the frequency. However, it takes a longer time to test, as the magnetic field must be generated in turn, and also, it takes a while until the magnetic field becomes stable because a response time is needed when timing the switching of the transmission coil or the like, so there has been a disadvantage that it is inefficient to test food that is being transferred on a conveyor belt.
On the other hand, to eliminate the abovementioned disadvantage of the metal detection device in which the frequency must be switched in turn, there has been a device to create a magnetic field with two different frequencies at the same time to detect metal (for example, see Patent Document 2).
In this case, the problem caused by switching time according to Patent Document 1 is eliminated. However, two frequencies—for example, the frequency signals of tens of kHz and hundreds of kHz—are applied to the same transmission coil, so it is necessary to set up a transmission coil driving circuit with a wide bandwidth, but a tuning circuit is not set up. Therefore, Q (=preserved energy/consumed energy=wL/R=mean frequency/bandwidth, wL is reactance by the transmission coil, and R is the resistance element of a tuning circuit) of a so-called tuning circuit was low, and driving efficiency was poor, so detection sensitivity was low. Low sensitivity may lead to a crucial problem in terms of safety, because the metal detection device tests part of the so-called ‘food quality,’ so it was hard to put this into practical use.    Non-patent Document 1: ‘Super Mepoli II the metal detection machine,’ by Shigeru Kubodera and seven other authors, Anritsu Technical, Anritsu Co. Ltd., Issued in July 1999, Issue No. 78, pages 69 to 75    Patent Document 1: Japanese Patent Publication No. S63-41502    Patent Document 2: Japanese Utility Model Publication No. Hei4-11187